1. Field of the Invention
The present invention relates generally to the field of electron beam imaging and deflection systems. More particularly, the present invention relates to an arrangement of a magnetic lens relative to magnetic compensation fields whereby the electron optical axis is shifted so as to be coincident with a deflected electron beam at all times.
2. Description of the Prior Art
Deflection units in prior art electron beam columns are known to have three basic configurations which provide deflection before the electron beam lands on a target. In general, the prior art configurations provide deflection before the final lens, deflection after the final lens or deflection within the final lens. In all of these configurations, a basic problem is that the beam does not land perpendicular to the target and off-axis aberrations are encountered.
Deflection before the final lens has the advantage that a short focal length or working distance can be used. A short focal length is desirable to provide high resolution capability and reduction of on-axis aberrations. A high resolution capability is mandatory for scanning electron microscopy uses. The disadvantages of deflection before the final lens are that the deflection angle and all deflection aberrations increase rapidly with increasing field coverage thereby limiting the operation to relatively small fields of view. Deflection before the final lens requires double deflection using two deflection yokes. The deflected beam is transmitted through the magnetic lens in a direction which is inclined to the electron optical axis of the magnetic lens which results in off-axis aberrations. An example of deflection before the lens is shown in the publication "A Computer-Controlled Electron-Beam Machine for Micro-Circuit Fabrication" by T. H. P. Chang and B. A. Wallman, Record of llth Symposium on Electron, Ion and Laser Beam Technology, San Francisco Press, Inc., at page 471.
Deflection after the final lens eliminates off-axis aberrations of the lens since the electron beam passes through the magnetic lens at the center of the lens prior to deflection. Deflection after the final lens results in a reduction of total deflection aberrations and permits a large field coverage. The disadvantage of deflection after the final lens is that a long focal length or working distance must be used which results in poor resolution because the use of a long focal length or working distance results in substantial on-axis aberrations. A further major disadvantage of deflection after the final lens is that the use of a long focal length or working distance necessitates a small opening angle of the beam which results in a very small beam current density in the target. The combination of poor resolution and small beam current is highly disadvantageous when the electron beam column is used to write patterns in semiconductor wafers having line widths less than about 1.5 micron.
U.S. Pat. No. 3,930,181 to H. C. Pfeiffer, assigned to the same assignee as the present application, describes an improved electron beam system wherein the electron beam magnetic deflection yoke is physically located inside the lens between the pole pieces of the final or projection magnetic lens of the electron beam column. The electron beam deflection apparatus of U.S. Pat. No. 3,930,181 is schematically illustrated in FIG. 2. The apparatus of the aforesaid patent is a compromise between the systems which use deflection before the lens and those which use deflection after the lens. The focal length or working distance of the lens can be chosen much shorter than that of the systems which use deflection after the final lens but not as short as with systems which use deflection before the final lens. The off-axis aberrations associated with deflection before the final lens are reduced in the aforesaid system as compared to the systems with deflection before the lens but are not as small as found in those systems which use deflection after the final lens. The beam current density is significantly improved over the systems using deflection after the final lens but the resolution is not as good as those systems which use deflection before the final lens.
Further, the electron beam does not land perpendicular to the target.
The present invention is distinct over the prior art in that the advantages of deflection before the final lens, after the final lens and within the final lens are combined in a single electron beam apparatus. The present invention utilizes deflection before the final lens which permits use of a short focal length or working distance but has the additional feature of being able to shift the electron optical axis of the final lens coincident with the deflected beam to eliminate off-axis aberrations associated with deflection before the final lens. The short focal length permits high resolution and high beam current. The movement of the electron optical axis permits large fields of view without the inherent disadvantages associated with deflection after the final lens.
The concept of providing a moving objective lens is disclosed in articles by H. Ohiwa, "Design of Electron-Beam Scanning Systems Using the Moving Objective Lens", J. Vac. Sci. Technol., 15 (3), May/June 1978, pp. 849-852 and E. Goto, "MOL (Moving Objective Lens)", Opik 48 (1977), pp. 255. In these articles some mathematical considerations for providing a moving objective lens are discussed. However, the articles do not suggest or disclose any working apparatus which could be used to provide a magnetic lens with a movable axis.